Intracellular TLR signaling: a structural perspective on human disease

Neutrophil Extracellular Traps-Associated RNA Impedes CD4+ Treg Differentiation by TLR7-IRF7 Axis in Ankylosing Spondylitis

OBJECTIVE

Our objective was to investigate the role of neutrophil extracellular traps (NETs) in the pathogenesis of inflammatory disorders in ankylosing spondylitis (AS).

METHODS

Local and circulating NETs levels were determined by immunofluorescence (IF) and myeloperoxidase (MPO)-DNA quantification in both patients with AS and AS model SKG mice. Flow cytometry (FCM) was performed to detect the effect of NETs on CD4+ subpopulation differentiation. The therapeutic effects of the neutrophil elastase inhibitor sivelestat (SVT) and the peptidylarginine deiminase 4 (PAD4) inhibitor CI-amidine were evaluated in SKG mice. The localization of NETs and their ability to impede CD4+ Treg cell differentiation were evaluated via IF, FCM, and Western blotting. RNA sequencing and specific inhibitors were used to clarify the detailed mechanism by which NETs inhibit CD4+ Treg differentiation.

RESULTS

The NETs levels were elevated locally and systemically in both patients with AS and SKG mice, which impeded the differentiation of CD4+ Treg cells. Blocking NETs formation via SVT or CI-amidine restored the CD4+ Treg ratio and subsequently alleviated inflammation in SKG mice. NETs were internalized by CD4+ T cells, and their associated RNA activated the Toll-like receptor 7 (TLR7)-interferon regulatory factor 7 (IRF-7) axis, which then inhibited Treg differentiation. Inhibiting CD4+ T cells endocytosis, removing the bound RNA component, or blocking the TLR7-IRF-7 axis abrogated the negative effect of NETs on CD4+ Treg differentiation.

CONCLUSION

Elevated NETs impeded CD4+ Treg differentiation by activating the TLR7-IRF-7 axis via their associated RNA in AS, and targeting NETs may be a novel treatment strategy for AS and related inflammatory disorders.

Emerging nanoparticle platforms for CpG oligonucleotide delivery

Unmethylated cytosine-phosphate-guanine (CpG) oligodeoxynucleotides (ODNs), which were therapeutic DNA with high immunostimulatory activity, have been applied in widespread applications from basic research to clinics as therapeutic agents for cancer immunotherapy, viral infection, allergic diseases and asthma since their discovery in 1995. The major factors to consider for clinical translation using CpG motifs are the protection of CpG ODNs from DNase degradation and the delivery of CpG ODNs to the Toll-like receptor-9 expressed human B-cells and plasmacytoid dendritic cells. Therefore, great efforts have been devoted to the advances of efficient delivery systems for CpG ODNs. In this review, we outline new horizons and recent developments in this field, providing a comprehensive summary of the nanoparticle-based CpG delivery systems developed to improve the efficacy of CpG-mediated immune responses, including DNA nanostructures, inorganic nanoparticles, polymer nanoparticles, metal-organic-frameworks, lipid-based nanosystems, proteins and peptides, as well as exosomes and cell membrane nanoparticles. Moreover, future challenges in the establishment of CpG delivery systems for immunotherapeutic applications are discussed. We expect that the continuously growing interest in the development of CpG-based immunotherapy will certainly fuel the excitement and stimulation in medicine research.

Mesenchymal stromal/stem cells and their exosomes application in the treatment of intervertebral disc disease: A promising frontier

Today, the application of mesenchymal stromal/stem cells (MSCs) and their exosomes to treat degenerative diseases has received attention. Due to the characteristics of these cells, such as self-renewability, differentiative and immunomodulatory effects, their use in laboratory and clinical studies shows promising results. However, the allogeneic transplantation problems of MSCs limit the use of these cells in the clinic. Scientists propose the application of exosomes to use from the therapeutic effect of MSCs and overcome their defects. These vesicles change the target cell behaviour and transcription profile by transferring various cargo such as proteins, mi-RNAs, and lipids. One of the degenerative tissue diseases in which MSCs and their exosomes are used in their treatment is intervertebral disc disease (IDD). Different factors such as genetics, nutrition, ageing, and environmental factors play a significant role in the onset and progression of this disease. These factors affect the cellular and molecular properties of the disc, leading to tissue destruction. Nucleus pulposus cells (NPCs) are among the most important cells involved in the pathogenesis of disc degeneration. MSCs exert their therapeutic effects by differentiating, reducing apoptosis, increasing proliferation, and decreasing senescence in NPCs. In addition, the use of MSCs and their exosomes also affects the annulus fibrosus and cartilaginous endplate cells in disc tissue and prevents disc degeneration progression.

Intratumoural immunotherapy: activation of nucleic acid sensing pattern recognition receptors

Recently, it has become clear that the tumour microenvironment (TME) is important in cancer immunotherapy. While immune checkpoint inhibitors are effective for some patients, the heterogeneous nature and status of the TME (‘cold’ tumours) play a critical role in suppressing antitumour immunity in non-responding patients. Converting ‘cold’ to ‘hot’ tumours through modulation of the TME may enable expansion of the therapeutic efficacy of immunotherapy to a broader patient population. This paper describes advances in intratumoural immunotherapy, specifically activation of nucleic acid sensing pattern recognition receptors to modulate the TME.

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Intratumoural immunotherapy to modulate the tumour microenvironment.

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Use of novel immunostimulatory agents which activate nucleic acid sensing pattern recognition receptors.

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Harnessing innate and adaptive immunity induced by receptor-mediated immune cascade.

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Intratumoural therapy leads to local and anenestic tumour responses.

A clinical association between Toll-like receptor 2 Arg753Gln polymorphism with recurrent cystic echinococcosis in postsurgery patients: A case control study

Recurrence of hydatid cysts in cystectomy patients has dramatically remained a serious concern within the surgical community. Predisposing factors for recurrence of hydatid cysts remained to be identified. Toll-like receptor (TLR) plays a pivotal role in bridging between acquired and innate immunity in cystic echinococcosis (CE) infection. 117 CE patients including 66 acute hydatidosis (AH; primary infection) and 51 recurrent hydatidosis (RH; chronic infection), and 117 ethnically matched healthy control (HC) were investigated from endemic regions of Iran in the period of 2015-2018. CE patients were definitely confirmed using histopathological and immunological assays. Genotyping of TLR2 Arg753Gln was carried out by restriction fragment length polymorphism and sequencing. The homozygous mutant-type TLR2 Gln/Gln (A/A) was represented to be associated with the occurrence of RH (P = 0.04) and conferred a 9 fold risk for susceptibility, while the heterozygous mutant-type TLR2 Arg/Gln (G/A) indicated a tendency to be associated with the occurrence of RH (P = 0.07). There was no discrepancy in the frequency of TLR2 Arg753Gln haplotypes between AH patients and HC individuals (P = 0.09). The mutant allele A was observed to be a risk factor for susceptibility to RH patients. Our results point to a clinical association between TLR2 Arg753Gln haplotypes with RH in postoperative patients. It can be inferred that allele G may lead to protection against the CE, while mutant allele A may be a diagnostic hallmark in the screening of RH susceptibility. Nevertheless, further studies with a larger sample size of different ethnic populations are required to authenticate this association.

TSG-6 secreted by bone marrow mesenchymal stem cells attenuates intervertebral disc degeneration by inhibiting the TLR2/NF-κB signaling pathway

Inflammation has been correlated with intervertebral disc degeneration (IDD). Recent evidence suggests that TNF-α-stimulated gene 6 protein (TSG-6) secreted by bone marrow mesenchymal stem cells (BMSCs) displays a remarkable ability to inhibit inflammatory processes in a variety of diseases. However, it is unknown whether BMSCs exert their therapeutic effect against IDD by secreting TSG-6. Here we investigated the effects of BMSCs and TSG-6 on IDD and explored the possible underlying mechanisms in vitro and in vivo. We found that BMSCs and TSG-6 reduced the expression of MMP-3 and MMP-13, and increased the expression of collagen II and aggrecan in the IL-1β-treated nucleus pulposus cells (NPCs), but the protective effects of BMSCs and TSG-6 were attenuated when TSG-6 expression was silenced. We also found that the activation of the TLR2/NF-κB pathway was inhibited by BMSCs and TSG-6. The levels of IL-6 and TNF-α in the degenerated NPCs were reduced and the proliferation of IL-1β-treated NPCs was increased in the presence of BMSCs and TSG-6. Furthermore, in vivo experiments showed that BMSCs and TSG-6 restored the MRI T2-weighted signal intensity and increased collagen II and aggrecan expression in the degenerated nucleus pulposus (NP) tissues. Finally, our results showed that BMSCs and TSG-6 downregulated the TLR2/NF-κB signaling and reduced the expression of MMPs and inflammatory cytokines in the degenerated NP tissues. The present study is the first to demonstrate the involvement of TLR2/NF-κB pathway in the potential anti-IDD therapeutic effect of TSG-6, and the results provide new insight into the beneficial effect of BMSCs in the treatment of IDD.

IL-1β/HMGB1 signalling promotes the inflammatory cytokines release via TLR signalling in human intervertebral disc cells

Inflammation and cytokines have been recognized to correlate with intervertebral disc (IVD) degeneration (IDD), via mediating the development of clinical signs and symptoms. However, the regulation mechanism remains unclear. We aimed at investigating the regulatory role of interleukin (IL)β and high mobility group box 1 (HMGB1) in the inflammatory response in human IVD cells, and then explored the signalling pathways mediating such regulatory effect. Firstly, the promotion to inflammatory cytokines in IVD cells was examined with ELISA method. And then western blot and real time quantitative PCR were performed to analyse the expression of toll-like receptors (TLRs), receptors for advanced glycation endproducts (RAGE) and NF-κB signalling markers in the IL-1β- or (and) HMGB1-treated IVD cells. Results demonstrated that either IL-1β or HMGB1 promoted the release of the inflammatory cytokines such as prostaglandin E2 (PGE2), TNF-α, IL-6 and IL-8 in human IVD cells. And the expression of matrix metalloproteinases (MMPs) such as MMP-1, -3 and -9 was also additively up-regulated by IL-1β and HMGB1. We also found such additive promotion to the expression of TLR-2, TLR-4 and RAGE, and the NF-κB signalling in intervertebral disc cells. In summary, our study demonstrated that IL-1β and HMGB1 additively promotes the release of inflammatory cytokines and the expression of MMPs in human IVD cells. The TLRs and RAGE and the NF-κB signalling were also additively promoted by IL-1β and HMGB1. Our study implied that the additive promotion by IL-1β and HMGB1 to inflammatory cytokines and MMPs might aggravate the progression of IDD.

The TLR2 and TLR4 gene polymorphisms in Moroccan visceral leishmaniasis patients

Visceral leishmaniasis (VL) is endemic in the Mediterranean basin and leads to the most severe form of Leishmania infection, lethal if left untreated. However, most infections are sub-clinical or asymptomatic, reflecting the influence of host genetic background on disease outcome. This study aimed to investigate possible association of TLR4 Asp299Gly, TLR4 Thr399Ile and TLR2 Arg753Gln polymorphisms with VL in Moroccan children. We enrolled 119 children with VL caused by Leishmania infantum as well as 138 unrelated children, 95 asymptomatic subjects and 43 healthy individuals who had no evidence of present or past infection. Polymorphisms were genotyped by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) and amplification refractory mutation system assay (ARMS-PCR). Results showed significant differences in genotype Thr399Ile and recessive model frequencies between VL and delayed-type hypersensitivity (DTH+) groups (p=0.018, OR=0.414CI 0.195-0.880; p=0.029, OR=0.448CI 0.214-0.938], respectively) by having the amino-acid threonine polymorphism as a reference in the VL group. Concerning the Asp299Gly there were a significant associations when comparing VL vs DTH+ (Asp299Gly genotype p=0.002, OR=0.326CI 0.158-0.671, allele frequencies p=0.033, OR=0.396CI 0.164-0.959, recessive model p=0.002, OR=0.343CI 0.172-0.681) and DTH+ vs DTH- groups (Asp299Gly genotype p=2.160E-4, OR=3.065CI 1.672-5.618, Gly299Gly genotype p=0.047, OR=0.368CI 0.299-0.452, allele frequencies p=1.406E-7, OR=29.571CI 3.907-223.8, recessive model p=4.370E-14, OR=36.965CI 8.629-158.3), by having the aspartic acid polymorphism as a reference these results suggest that the allele A (savage) confer protection against the clinical manifestations but not against the infection. Furthermore, there was a significant association regarding the Arg753Gln genotype (p=0.002, OR=0.326CI 0.158-0.671), allele frequencies (p=0.033, OR=0.396CI 0.164-0.959) and when applying a recessive model (p=0.002, OR=0.343CI 0.172-0.681) in the VL vs DTH+ groups. The same results was observed when comparing DTH+ vs DTH- groups (p=4.136E-6, OR=0.211CI 0.104-0.428), allele frequencies (p=0.008, OR=0.327CI 0.137-0.779) and recessive model (p=1.748E-5, OR=0.244CI 0.124-0.480). The results provide evidence that allele C in Thr399Ile and allele G in Arg753Gln polymorphisms may lead to protection against the clinical disease. Our data provide insights into the possible role of TLR2 and TLR4 variations in VL susceptibility.

The interaction of lubricin/proteoglycan 4 (PRG4) with toll-like receptors 2 and 4: an anti-inflammatory role of PRG4 in synovial fluid

Background

Lubricin/proteoglycan-4 (PRG4) is a mucinous glycoprotein secreted by synovial fibroblasts and superficial zone chondrocytes. PRG4 has a homeostatic multifaceted role in the joint. PRG4 intra-articular treatment retards progression of cartilage degeneration in pre-clinical posttraumatic osteoarthritis models. The objective of this study is to evaluate the binding of recombinant human PRG4 (rhPRG4) and native human PRG4 (nhPRG4) to toll-like receptors 2 and 4 (TLR2 and TLR4) and whether this interaction underpins a PRG4 anti-inflammatory role in synovial fluid (SF) from patients with osteoarthritis (OA) and rheumatoid arthritis (RA).

Methods

rhPRG4 and nhPRG4 binding to TLR2 and TLR4 was evaluated using a direct enzyme linked immunosorbent assay (ELISA). Association of rhPRG4 with TLR2 and TLR4 overexpressing human embryonic kidney (HEK) cells was studied by flow cytometry. Activation of TLR2 and TLR4 on HEK cells by agonists Pam3CSK4 and lipopolysaccharide (LPS) was studied in the absence or presence of nhPRG4 at 50, 100 and 150 μg/ml. Activation of TLR2 and TLR4 by OA SF and RA SF and the effect of nhPRG4 SF treatment on receptor activation was assessed. PRG4 was immunoprecipitated from pooled OA and RA SF. TLR2 and TLR4 activation by pooled OA and RA SF with or without PRG4 immunoprecipitation was compared.

Results

rhPRG4 and nhPRG4 exhibited concentration-dependent binding to TLR2 and TLR4. rhPRG4 associated with TLR2- and TLR4-HEK cells in a time-dependent manner. Co-incubation of nhPRG4 (50, 100 and 150 μg/ml) and Pam3CSK4 or LPS reduced TLR2 or TLR4 activation compared to Pam3CSK4 or LPS alone (p <0.05). OA SF and RA SF activated TLR2 and TLR4 and nhPRG4 treatment reduced SF-induced receptor activation (p <0.001). PRG4 depletion by immunoprecipitation significantly increased TLR2 activation by OA SF and RA SF (p <0.001).

Conclusion

PRG4 binds to TLR2 and TLR4 and this binding mediates a novel anti-inflammatory role for PRG4.

Mesenchymal stem cells secrete immunologically active exosomes

Mesenchymal stem cells (MSCs) have been shown to secrete exosomes that are cardioprotective. Here, we demonstrated that MSC exosome, a secreted membrane vesicle, is immunologically active. MSC exosomes induced polymyxin-resistant, MYD88-dependent secreted embryonic alkaline phosphatase (SEAP) expression in a THP1-Xblue, a THP-1 reporter cell line with an NFκB-SEAP reporter gene. In contrast to lipopolysaccharide, they induced high levels of anti-inflammatory IL10 and TGFβ1 transcript at 3 and 72 h, and much attenuated levels of pro-inflammatory IL1B, IL6, TNFA and IL12P40 transcript at 3-h. The 3-h but not 72-h induction of cytokine transcript was abrogated by MyD88 deficiency. Primary human and mouse monocytes exhibited a similar exosome-induced cytokine transcript profile. Exosome-treated THP-1 but not MyD88-deficient THP-1 cells polarized activated CD4(+) T cells to CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) at a ratio of one exosome-treated THP-1 cell to 1,000 CD4(+) T cells. Infusion of MSC exosomes enhanced the survival of allogenic skin graft in mice and increased Tregs.

Proteomics reveals acute pro-inflammatory and protective responses in rat Kupffer cells and hepatocytes after chemical initiation of liver cancer and after LPS and IL-6

Inflammation is a key event in the development of liver cancer. We studied early inflammatory responses of Kupffer cells (KCs) and hepatocyte (HC) after cancer initiation. The chemical carcinogen N-nitrosomorpholine (NNM) was used in a rat model. We applied a comprehensive analytical strategy including metabolic labeling, 2-D PAGE, LC-MS/MS-based spot identification and shotgun proteomics and thus determined the rates of synthesis of individual proteins, compared whole tissue with isolated constituent cells and performed in vivo to in vitro comparisons of NNM effects. NNM increased synthesis of overall and 138 individual proteins identified in HC and/or KC, indicating reprogramming of metabolism favoring protection, repair and replacement of cell constituents in HC and KC. Secretome analysis by 2-D PAGE and shotgun proteomics of HC revealed the induction of acute phase proteins, in case of KC of proteases, cytokines and chemokines, indicating inflammatory effects. All responses were induced rapidly, independently of signals from other cells, and closely mimicked the pro-inflammatory and protective effects of inflammation modulators LPS in KC and IL-6 in HC. In conclusion, the carcinogen NNM exerts pro-inflammatory effects in the liver, partially by direct activation of KC. The acute inflammation and its protective component will enhance formation, survival and proliferation of initiated cells and may therefore act synergistically with the genotoxic action of the carcinogen.

Mycobacterium bovis bacilli Calmette-Guerin regulates leukocyte recruitment by modulating alveolar inflammatory responses

Leukocyte migration into the epithelial compartment is an important feature in the active phase of mycobacterial infections. In this study, we used the Transwell model to investigate the mechanisms behind mycobacteria-induced leukocyte recruitment and investigated the role of TLR2 and TLR4 in this process. Infection of epithelial cells resulted in significantly increased secretion of the neutrophil chemotactic CXCL8 and IL-6, but no secretion of monocyte chemotactic CCL2 or TNF-α was observed. In contrast to epithelial response, mycobacteria-infected neutrophils and monocytes secreted all these cytokines. Corresponding with epithelial cytokine response, mycobacterial infection of the epithelial cells increased neutrophil diapedesis, but decreased monocyte recruitment. However, monocyte recruitment towards mycobacteria infected epithelial cells significantly increased following addition of neutrophil pre-conditioned medium. Mycobacterial infection also increases alveolar epithelial expression of TLR2, but not TLR4, as analyzed by flow cytometry, Western blotting and visualized by confocal microscopy. Blocking of TLR2 inhibited neutrophil recruitment and cytokine secretion, while blocking of TLR4 had a lesser effect. To summarize, we found that primary alveolar epithelial cells produced a selective TLR2-dependent cytokine secretion upon mycobacterial infection. Furthermore, we found that cooperation between cells of the innate immunity is required in mounting proper antimicrobial defence.

Intrinsic danger: activation of Toll-like receptors in rheumatoid arthritis

RA is a debilitating disorder that manifests as chronic localized synovial and systemic inflammation leading to progressive joint destruction. Recent advances in the molecular basis of RA highlight the role of both the innate and adaptive immune system in disease pathogenesis. Specifically, data obtained from in vivo animal models and ex vivo human tissue explants models has confirmed the central role of Toll-like receptors (TLRs) in RA. TLRs are pattern recognition receptors (PRRs) that constitute one of the primary host defence mechanisms against infectious and non-infectious insult. This receptor family is activated by pathogen-associated molecular patterns (PAMPs) and by damage-associated molecular patterns (DAMPs). DAMPs are host-encoded proteins released during tissue injury and cell death that activate TLRs during sterile inflammation. DAMPs are also proposed to drive aberrant stimulation of TLRs in the RA joint resulting in increased expression of cytokines, chemokines and proteases, perpetuating a vicious inflammatory cycle that constitutes the hallmark chronic inflammation of RA. In this review, we discuss the signalling mechanisms of TLRs, the central function of TLRs in the pathogenesis of RA, the role of endogenous danger signals in driving TLR activation within the context of RA and the current preclinical and clinical strategies available to date in therapeutic targeting of TLRs in RA.

Influence of polymorphisms in innate immunity genes on susceptibility to invasive aspergillosis after stem cell transplantation

The innate immune system plays a pivotal role in the primary defence against invasive fungal infection. Genetic variation in genes that regulate this response, initiated by pulmonary macrophages, may influence susceptibility to invasive aspergillosis in patients at risk. We investigated in a clinical setting whether common polymorphisms in Toll-like receptor (TLR) and cytokine genes involved in macrophage regulation are associated with susceptibility to invasive aspergillosis. Forty-four allogeneic stem cell transplantation recipients diagnosed with probable or proven IA according to the criteria of the European Organization for Research and Treatment of Cancer/Mycoses Study Group, were enrolled. The control group consisted of 64 allogeneic stem cell transplantation recipients without invasive aspergillosis. The TLR4 1063A>G single nucleotide polymorphism was associated with invasive aspergillosis when present in donors of allogeneic stem cell transplantation recipients (unadjusted OR 3.77 95%CI 1.08–13.2, p = 0.03). In a multivariate analysis, adjusted for occurrence of graft-versus-host-disease, Cytomegalovirus serostatus and duration of neutropenia, paired presence of the TLR4 1063A>G and IFNG 874T>A single nucleotide polymorphisms showed a trend towards increased susceptibility to invasive aspergillosis (p = 0.04). These findings point to the relevant immunological pathway involved in resistance to invasive aspergillosis and warrant further study of the effects of TLR and cytokine polymorphisms and their interaction, which may occur on different levels of the complex biological interplay between the immunocompromised host and Aspergillus sp.

Cracking the Toll-like receptor code in fungal infections

Innate control of fungal infection requires the specific recognition of invariant fungal molecular structures by a variety of innate immune receptors, including Toll-like receptors. In addition to the role in inducing protective immune responses, Toll-like receptor engagement may paradoxically favor fungal infections, by inducing inflammatory pathology and impairing antifungal immunity. Although the dissection of complex genetic traits modulating susceptibility to fungal infections is complex, the contribution of host genetics may hold the key to elucidating new risk factors for these severe, often fatal diseases. Understanding host-pathogen interactions at the innate immune interface will eventually lead to the development of new therapeutics and genetic markers in fungal infections.

Heat shock protein 90 protects rat mesenchymal stem cells against hypoxia and serum deprivation-induced apoptosis via the PI3K/Akt and ERK1/2 pathways

Mesenchymal stem cell (MSC) transplantation has shown a therapeutic potential to repair the ischemic and infracted myocardium, but the effects are limited by the apoptosis and loss of donor cells in host cardiac microenvironment. The aim of this study is to explore the cytoprotection of heat shock protein 90 (Hsp90) against hypoxia and serum deprivation-induced apoptosis and the possible mechanisms in rat MSCs. Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Apoptosis was assessed by Hoechst 33258 nuclear staining and flow cytometric analysis with annexin V/PI staining. The gene expression of Toll-like receptor-4 (TLR-4) and V-erb-b2 erythroblastic leukemia viral oncogene homolog 2 (ErbB2) was detected by real-time polymerase chain reaction (PCR). The protein levels of cleaved caspase-3, Bcl-2, Bcl-xL, Bax, total-ERK, phospho-ERK, total-Akt, phospho-Akt, and Hsp90 were detected by Western blot. The production of nitric oxide was measured by spectrophotometric assay. Hsp90 improves MSC viability and protects MSCs against apoptosis induced by serum deprivation and hypoxia. The protective role of Hsp90 not only elevates Bcl-2/Bax and Bcl-xL/Bax expression and attenuates cleaved caspase-3 expression via down-regulating membrane TLR-4 and ErbB2 receptors and then activating their downstream PI3K/Akt and ERK1/2 pathways, but also enhances the paracrine effect of MSCs. These findings demonstrated a novel and effective treatment strategy against MSC apoptosis in cell transplantation.

Genetic susceptibility to aspergillosis in allogeneic stem-cell transplantation

Invasive aspergillosis (IA) is a major threat to positive outcomes for allogeneic stem-cell transplantation (allo-SCT) patients. Despite presenting similar degrees of immunosuppression, not all individuals at-risk ultimately develop infection. Therefore, the traditional view of neutropenia as a key risk factor for aspergillosis needs to be accommodated within new conceptual advances on host immunity and its relationship to infection. Polymorphisms in innate immune genes, such as those encoding TLRs, cytokines and cytokine receptors, have recently been associated with susceptibility to IA in allo-SCT recipients. This suggests that understanding host-pathogen interactions at the level of host genetic susceptibility will allow the formulation of new targeted and patient-tailored antifungal therapeutics, including improved donor screening.

Lipids from attenuated and virulent Babesia bovis strains induce differential TLR2-mediated macrophage activation

Babesia bovis is an intraerythrocytic apicomplexan protozoa of cattle that causes an acute infection with parasite persistence. Babesiosis limitation depends on macrophages, essential effector cells of the host innate defense, which generate inflammatory cytokines and nitric oxide. Herein, we report quantitative differences in the lipid composition of merozoites from two B. bovis strains with polar behaviour: attenuated R1A and virulent S2P. Accordingly, we observed a distinct inflammatory response induced by the total lipids of R1A (L(A)) and S2P (L(V)) in murine peritoneal macrophages. L(A) and particularly its fractions phosphatidic acid and phosphatidylserine+phosphatidylinositol (PS+PI), produced a strong activation of these cells with lipid body formation, cyclooxygenase-2 expression and pro-inflammatory TNFalpha, IL-6 and KC secretion. Although L(V) did not activate these cells, the corresponding PS+PI fraction induced TNFalpha, IL-6 and KC release. Therefore, these facts might be suggesting the presence of an inhibitor in L(V). Furthermore, the employment of wild type and toll like receptor 2 knockout (TLR2KO) mice allowed us to demonstrate that macrophage activation by the stimulating lipid fractions was mediated through TLR2. Interestingly, only L(A) activated the extracellular signal-regulated kinases 1 and 2 (ERK1/2). Inhibitory studies employing UO126, indicated that the ERK pathway was required for TNFalpha, IL-6 and KC release. In conclusion, the absence of inflammatory response observed with the lipids of S2P virulent strain could constitute an evasion mechanism of the innate immune response enabling parasite establishment in the host.

Effector mechanisms in HIV-1 infected elite controllers: highly active immune responses?

Elite controllers (EC) are HIV-1 infected patients control viral replication to a level of <50 copies/ml without antiretroviral therapy. These patients are also known as elite suppressors, or HIV controllers, and they differ from traditional long-term non-progressors (LTNPs) who maintain stable CD4 counts and are asymptomatic without antiretroviral therapy. Recent studies suggest that many EC are infected with replication-competent virus. Thus it appears that host factors such as innate immunity, the humoral immune response, and the cellular immune response are involved in the suppression of viral replication in EC. This article will review the effector mechanisms that are thought to play a role in the remarkable control of viral replication seen in these patients. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

Toll-like receptor 4 signaling pathway can be triggered by grass carp reovirus and Aeromonas hydrophila infection in rare minnow Gobiocypris rarus

Toll-like receptor 4 (TLR4) is critical for LPS recognition and cellular responses. It also recognizes some viral envelope proteins. Detection mostly results in the inflammation rather than specific antiviral responses. However, it's unclear in fish. In this report, a TLR4 gene (named as GrTLR4b) was cloned and characterized from rare minnow Gobiocypris rarus. The full length of GrTLR4b cDNA consists of 2766 nucleotides and encodes a polypeptide of 818 amino acids with an estimated molecular mass of 94,518Da and a predicted isoelectric point of 8.41. The predicted amino acid sequence comprises a signal peptide, six leucine-rich repeat (LRR) motifs, one leucine-rich repeat C-terminal (LRRCT) motif, followed by a transmembrane segment of 23 amino acids, and a cytoplasmic region of 167 amino acids containing one Toll--interleukin 1--receptor (TIR) motif. It's closely similar to the zebrafish (Danio rerio) TLR4b amino acid sequence with an identity of 77%. Quantitative RT-PCR analysis showed GrTLR4b mRNA was constitutive expression in gill, heart, intestine, kidney, liver, muscle and spleen tissues in healthy animals and up-regulated by viruses and bacteria. After being infected by grass carp reovirus or Aeromonas hydrophila, GrTLR4b expressions were up-regulated from 24h post-injection and lasted until the fish became moribund (P<0.05). These data implied that TLR4 signaling pathway could be activated by both viral and bacterial infection in rare minnow.

Polymorphisms in Toll-like receptor genes and susceptibility to infections in allogeneic stem cell transplantation

OBJECTIVE

Discovery of genetic variations in the genes encoding for Toll-like receptors (TLRs) has highlighted a potential link between genomic variation of the host and susceptibility to infections.

MATERIALS AND METHODS

We investigated the association between polymorphisms in the TLR2, TLR4, and TLR9 genes in recipients of allogeneic hematopoietic stem cell transplant and susceptibility to infections caused by cytomegalovirus and filamentous fungi.

RESULTS

A significant association was observed between the presence of the T-1237C polymorphism (TLR9) and susceptibility to viral pneumonia (p=0.04; odds ratio [OR]: 1.73). For fungi, a significant association was observed between the presence of the cosegregating Asp299Gly/Thr399Ile polymorphisms (TLR4) and fungal colonization (p=0.003; OR: 10.6). However, susceptibility to fungal infections, predominantly fungal pneumonia, was instead significantly decreased in the presence of the same polymorphisms (p=0.03; OR: 0.23).

CONCLUSION

Thus, fungal colonization may not predict susceptibility to infection in the presence of these single nucleotide polymorphisms. The finding that defective viral but not fungal sensing may predict susceptibility to infection highlights the divergent function of TLRs in the pathogenesis of opportunistic infections.

Structure and regulation of cytoplasmic adapter proteins involved in innate immune signaling

Initiation of the innate immune response requires agonist recognition by a pathogen recognition receptor. Following ligand binding, conformational rearrangement of the receptor creates a molecular scaffold from which signal transduction is propagated via complex cellular signaling pathways. This in turn leads to the induction of a pro-inflammatory immune response. A critical component of these signaling pathways is the homotypic interaction of receptor and adapter proteins via specific protein interaction domains. Within the innate immune signaling cascade, homotypic interactions between members of the death domain family and the Toll/interleukin-1 receptor domain are particularly important. Here we discuss the current understanding of the molecular basis of these homotypic receptor:adapter interactions and their role in innate immune signal transduction.

Toll-like receptor genes are differentially expressed at the sites of infection during the progression of Johne's disease in outbred sheep

Toll-like receptors (TLR) are engaged by ligands on microbial pathogens to initiate innate and adaptive immune responses. Little is known about TLR involvement during infection with Mycobacterium avium subsp. paratuberculosis (M. ptb), the cause of Johne's disease in ruminants, although there is a profound immunopathological response in affected animals. We have analyzed the expression of 10 TLR genes relative to validated reference genes at predilection sites in ileum, jejunum and associated lymph nodes as well as in peripheral blood, to determine if TLR expression is altered in response to infection with M. ptb in outbred sheep. Previously unexposed animals from two flocks and animals from three naturally infected flocks were used with restricted maximum likelihood linear mixed modeling applied to determine significant differences. These were related to the pathologies observed at different stages of infection in exposed sheep, after allowing for other sources of variation. In most cases there were differences in TLR expression between early paucibacillary and multibacillary groups when compared to uninfected sheep, with most TLRs for the paucibacillary group having lower expression levels than the multibacillary group. Increased expression of TLR1-5, and 8 was observed in ileum or jejunum, and TLR1-4, 6, and 8 in mesenteric lymph nodes. There was a trend for increased expression of TLR1, 2, and 6-8 in PBMCs of exposed compared to non-exposed animals. Further study of TLR expression in Johne's disease in ruminants is warranted as these observed differences may help explain pathogenesis and may be useful in the future diagnosis of M. ptb infection.

Signalling of toll-like receptors

Since Toll-like receptor (TLR) signaling was found crucial for the activation of innate and adaptive immunity, it has been the focus of immunological research. There are at least 13 identified mammalian TLRs, to date, that share similarities in their extracellular and intracellular domains. A vast number of ligands have been identified that are specifically recognized by different TLRs. As a response the TLRs dimerize and their signaling is initiated. The molecular basis of that signaling depends on the conserved part of their intracellular domain; namely the Toll/IL-1 receptor (TIR) domain. Upon TLR dimerization a TIR-TIR structure is formed that can recruit TIR-containing intracellular proteins that mediate their signaling. For this reason these proteins are named adapters. There are five adapters identified so far named myeloid differentiation primary response protein 88 (MyD88), MyD88-adapter like (Mal) or TIR domain-containing adapter (TIRAP), TIR domain-containing adapter inducing interferon-beta (IFN-beta) (TRIF) or TIR-containing adapter molecule-1 (TICAM-1), TRIF-related adapter molecule (TRAM) or TICAM-2, and sterile alpha and HEAT-Armadillo motifs (SARM). The first four play a fundamental role in TLR-signaling, defining which pathways will be activated, depending on which of these adapters will be recruited by each TLR. Among these adapter proteins MyD88 and TRIF are now considered as the signaling ones and hence the TLR pathways can be categorized as MyD88-dependent and TRIF-dependent.

Synthetic agonists of Toll-like receptors 7, 8 and 9

TLRs (Toll-like receptors) are a family of innate immune receptors that induce protective immune responses against infections. Single-stranded viral RNA and bacterial DNA containing unmethylated CpG motifs are the ligands for TLR7 and TLR8 and 9 respectively. We have carried out extensive structure–activity relationship studies of DNA- and RNA-based compounds to elucidate the impact of nucleotide motifs and structures on these TLR-mediated immune responses. These studies have led us to design novel DNA- and RNA-based compounds, which act as potent agonists of TLR9 and TLR7 and 8 respectively. These novel synthetic agonists produce different immune response profiles depending on the structures and nucleotide motifs present in them. The ability to modulate TLR-mediated immune responses with these novel DNA- and RNA-based agonists in a desired fashion may allow targeting a broad range of diseases, including cancers, asthma, allergies and infections, alone or in combination with other therapeutic agents, and their use as adjuvants with vaccines. IMO-2055, our first lead candidate, is a TLR9 agonist that is currently in clinical evaluation in oncology patients. A second candidate, IMO-2125, is also a TLR9 agonist that has been shown to induce high and sustained levels of IFN (interferon) in non-human primates and is being evaluated in HepC-infected human subjects.

Infections and other inflammatory conditions

The etiology of cerebral palsy and other related perinatal brain injuries is poorly understood. Infections of the central nervous system are rare but important causes of neurodisability. Recent evidence suggests that infections and other inflammatory conditions apparently limited to the placenta are also associated with an increased risk of neurologic impairment. A major hypothesis to explain this connection is that cytokines, activated inflammatory cells, and other mediators of the innate immune response are released into the fetal circulation where they can directly or indirectly affect the development or integrity of the central nervous system. This review surveys the organisms, mediators, and placental lesions that have been associated with perinatal brain injury.

Intracellular signalling cascades regulating innate immune responses to Mycobacteria: branching out from Toll-like receptors

Toll-like receptors (TLRs) recognize Mycobacterium tuberculosis (Mtb) or Mtb components and initiate mononuclear phagocyte responses that influence both innate and adaptive immunity. Recent studies have revealed the intracellular signalling cascades involved in the TLR-initiated immune response to mycobacterial infection. Although both TLR2 and TLR4 have been implicated in host interactions with Mtb, the relationship between specific mycobacterial molecules and various signal transduction pathways is not well understood. This review will discuss recent studies indicating critical roles for mycobacteria and mycobacterial components in regulation of mitogen-activated protein kinases and related signal transduction pathways that govern the outcome of infection and antibacterial defence. To better understand the roles of infection-induced signalling cascades in molecular pathogenesis, future studies are needed to clarify mechanisms that integrate the multiple signalling pathways that are activated by engagement of TLRs by both individual mycobacterial molecules and whole mycobacteria. These efforts will allow for the development of novel diagnostic and therapeutic modalities for tuberculosis that targets the intracellular signalling pathways permitting the replication of this nefarious pathogen.

Characterization of a TIR-like protein from Paracoccus denitrificans

Based on protein sequence homology searches, we found a conserved open reading frame within the genome of several human pathogenic bacteria showing a resemblance to the mammalian TIR domain. We cloned, expressed, and characterized the corresponding gene product from Paracoccus denitrificans using several biophysical techniques. The protein consists of two independently folded domains. As predicted from the amino acid sequence and experimentally confirmed here, the N-terminal domain consists of a alpha-helical coiled-coil. The NMR data indicates that the C-terminal TIR-like domain folds into a compact protein. Finally, using GST pull-down experiments, we show that the bacteria TIR-like domain binds to the mammalian receptor (TLR4) and adaptor (MyD88) TIR domains. We postulate that prokaryotic pathogens utilize the TIR-like proteins to interfere with the innate immune response of the mammalian host so that the bacterial infection can progress undetected.

The T-cell antigen receptor: a logical response to an unknown ligand

The immune system can be roughly divided into innate and adaptive compartments. The adaptive compartment includes the B and T lymphocytes, whose antigen receptors are generated by recombination of gene segments. The consequence is that the creation of self-reactive lymphocytes is unavoidable. For the host to remain viable, the immune system has evolved a strategy for removing autoimmune lymphocytes during development. This review discusses how T lymphocytes are generated, how they recognize antigens, and how their antigen receptor directs the removal of self-reactive T cells.

Members of the Toll-like receptor family of innate immunity pattern-recognition receptors are abundant in the male rat reproductive tract

Protecting developing and maturing spermatozoa and reproductive tissues from microbial damage is an emerging aspect of research in reproductive physiology. Bacterial, viral, and yeast infections of the testis and epididymis can hinder maturation and movement of spermatozoa, resulting in impaired fertility. Toll-like receptors (TLRs) are a broad family of innate immunity receptors that play critical roles in detecting and responding to invading pathogens. Objectives of this study were to determine if organs of the rat male reproductive tract express mRNAs for members of the TLR family, to characterize expression patterns for TLRs in different regions of the epididymis, and to determine if TLR adaptor and target proteins are present in the male reproductive tract. Messenger RNA for Tlr1-Tlr9 was abundantly expressed in testis, epididymis, and vas deferens, as determined by RT-PCR, while Tlr10 and Tlr11 were less abundantly expressed. Tlr mRNA expression showed no region-specific patterns in the epididymis. Immunoblot analysis revealed relatively equal levels of protein for TLRs 1, 2, 4, and 6 in testis, all regions of the epididymis and vas deferens, and lower levels of TLRs 3, 5, and 9-11. TLR7 was primarily detected in the testis. The TLR adapter proteins, myeloid differentiation primary response gene 88 and TLR adaptor molecule 1, as well as v-rel reticuloendotheliosis viral oncogene homolog and NFKBIA, were prominent in testis, epididymis, and vas deferens. The abundant expression of a majority of TLR family members together with expression of TLR adaptors and activation targets provides strong evidence that TLRs play important roles in innate immunity of the male reproductive tract.

Innate immunity: toll-like receptors and some more. A brief history, basic organization and relevance for the human newborn

The discovery of Toll-like receptors (TLRs) as essential components of the innate immune system has greatly advanced our knowledge and understanding of immune responses to infection and how these are regulated. Innate immunity in general and TLRs in particular play a crucial role in the front line of host defenses against microbes, but also are a key element in the proper functioning of the immune system at large in vertebrate animals. The innate immune system has been identified as a collection of factors, both cell-associated and cell-free, that comprises an impressively effective and well-organized system that is capable of immediate recognition of a whole array of microbes and microbial components. The cell-bound TLRs fulfill a central role in the process from pathogen recognition to activation of adaptive immunity. From the cell-free factors the plasma protein mannose-binding lectin (MBL) has been studied most extensively. Associations have already been documented between TLR polymorphisms in man and TLR deficiency in animals and an increased susceptibility to infection. The effect of MBL on infectious disease susceptibility only seems to emerge when host defenses are compromised by a severe underlying condition. The functional state of the various components of innate immunity at birth is largely unknown and only recently a number of studies have assessed this feature of the innate immune system. In addition, for the human newborn the innate immune system may have a broader significance; it may well be the key system determining the course of inflammatory events associated with premature birth, a notion that is emphasized by the recently described association between TLR polymorphisms and prematurity. However, there are still many open questions, particularly about the exact relation between individual TLRs and infectious disease susceptibility and how TLRs cooperate in resistance to infection and in initiating adaptive immune responses. With regard to the human newborn, the most relevant question that needs to be resolved is the precise role of innate immunity in the pathogenesis of prematurity.